Advances in semiconductor processing and device design have resulted in computing devices being incorporated with a seemingly endless variety of tools and machines ranging from conventional programmable computers to communication equipment and entertainment devices. Irrespective of its end purpose, a computing device is generally made up of three main components, a central processing unit (CPU), random access memory (RAM), and an addressing and communication mechanism enabling data exchange between the CPU and the RAM. In a computing device, data is stored, communicated and manipulated in the form of signals (e.g., voltage, current, optical and the like). The CPU contains circuitry for logically manipulating the signals whereas the memory contains circuitry for storing the signals before, during and after the CPU's manipulation of those signals.
Conventional CPUs, memory devices and data communication mechanisms are mass produced as solid state electronic devices. Although sometimes referred to as “semiconductor devices”, solid state electronic devices rely on electrical behavior of solids including metals, semiconductors, and insulators. The techniques and equipment for producing solid state devices have improved dramatically over time to enable the production of devices such as switches, capacitors, resistors, and interconnections with sub-micron scale features.
Performance of the memory components of a computing device is becoming an increasingly important determinant of overall system performance. Larger quantities of memory enable a greater variety of applications and functions to be implemented by the computing device and may reduce or eliminate the need for separate mass storage devices. Higher speed memory supports higher CPU processing frequencies, making the computing devices more useful for complex or real-time tasks. Denser memory devices support a growing variety of battery-powered electronic devices such as laptop computers, PDAs, multifunction cellular telephones, and the like. At the same time, many of these applications benefit from reduced power consumption.
In many cases improvements in semiconductor processing technology have had the effect of improving each of these important figures to make denser, larger, faster and more power efficient memory devices. In many cases, the solid state electronic behavior of the devices improves as the devices become smaller. Unfortunately, conventional silicon-based memory, such as DRAM memory, has reached a point where continued reduction in the size of conventional semiconductor memory cells is expected to adversely affect at least some of these important parameters.
One reason for the reduced speed and increased power consumption at smaller dimensions is that memory devices usually implement a capacitor for each stored bit of information. A capacitor is a charge storage device formed by conductive plates that are separated by an insulator. As capacitors become smaller the quantity of charge that can be stored is reduced. To serve as a reliable memory device, a capacitor must have sufficient capacity to hold a signal at a level that can be later reliable detected as data. Moreover, capacitors are inherently “leaky” devices in that some of the charge stored in a capacitor dissipates or leaks over time. Memory devices based on smaller capacitors are more sensitive to leakage problems because they simply have less charge that can be lost before the stored data becomes irretrievable.
To overcome the transient nature of capacitive storage, memory devices use refresh circuitry that frequently reads out a stored signal, amplifies it to a higher level, and stores it back into the capacitor. As a capacitors shrink, the rate at which the capacitor needs to be refreshed increases. In turn, higher capacitor refresh rates reduce the percentage of time that a memory cell is available for reading and writing data. Moreover, a greater percentage of the total power consumption of the memory device is then used to refresh the memory. Even when the device is in a dormant or inactive state, traditional DRAM requires continuous refreshing and therefore continuous power consumption. Accordingly, researchers are actively seeking new ways of storing data signals that overcome the problems associated with smaller capacitors in conventional capacitor based memory devices.
Memory cell designers have attempted to maintain low refresh rates in smaller memory cells by boosting the amount of capacitance that can be formed in a given amount of chip area. Boosting capacitance often involves increasing the surface area of the capacitor's charge holding material, which is very difficult to do when the overall size of the capacitor is shrinking. While designers have had some success at controlling surface area by forming the charge holding material into three-dimensional trench and stacked capacitor designs, it is unlikely that these techniques can be relied for continued progress rendering larger capacitances in smaller devices. The solid state electronic behaviors upon which device performance is predicated begin to break down as the dimensions of various device features become smaller such that a capacitor can no longer store sufficient charge for sufficient time to be useful in a memory device.
Another problem facing memory designers trying to increase information density (e.g., the amount of information that can be stored in a given area of the memory chip). Each memory cell of a conventional solid state capacitor can only store one bit of information. Accordingly, it would be desirable to have a memory device with improved information storage density achieved by having a memory cell that can reliably store a plurality of discrete states.
In view of the above, it is apparent that a need exists for memory devices, such as dynamic random access memory, that overcome limitations imposed by conventional solid state memory design. In particular, there is a need for molecular memory cells, molecular memory arrays, and electronic devices including molecular memory. Further, there is a need for molecular memory devices that can be manufactured using techniques that are compatible with existing semiconductor manufacturing practices so that semiconductor devices and interconnections can be manufactured monolithically with molecular memory devices.